1. Field of the Invention
The present invention relates generally to characterization of the optical performance of a projection imaging system, in particular, the measurement of the exit pupil transmittance of a projection imaging system.
2. Background
Improving the performance of existing and future steppers can have a large impact on the economics of projection imaging systems, such as those used in the production of microchips and flat panel displays. There has been some development of techniques to improve projection imaging systems through minimally intrusive retrofitting. See McArthur et al., “Plate Correction of Imaging Systems”, U.S. Pat. No. 5,392,119, Feb. 21, 1995; McArthur et al., “Plate Correction Technique for Imaging Systems”, U.S. Pat. No. 5,640,233, Jan. 26, 1995; McArthur et al., “Single Plate Corrector for Stepper Lens Train”, U.S. Pat. No. 5,929,991, Jul. 27, 1999; and Smith et al., “Apparatus Method of Measurement and Method of Data Analysis for Correction of Optical Systems”, U.S. Pat. No. 5,978,085, Nov. 2, 1999; MacDonald et al., “Imaging and Illumination System with Aspherization and Aberration Correction by Phase Steps”, U.S. Pat. No. 5,136,413, Aug. 4, 1992. Also in-situ interferometer techniques (see A. Smith et al., “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System”, U.S. Pat. No. 5,828,455, Oct. 27, 1998, “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System”, supra), and source metrology techniques (see McArthur et al., “In-Situ Source Metrology Instrument and Method Use”, U.S. Pat. No. 6,356,345, Mar. 12, 2002) have been used to measure projection imaging systems so that they may be improved or adjusted. In addition, recent advances in mask making may be utilized to compensate for transmittance profiles.
In order to adjust a projection imaging system, it is helpful to be able to quickly and reproducibly monitor the state of optical performance of the imaging system. In the above references, distortion and field curvature data from exposed images are inferred, and used to design figured optical surfaces that may be placed between the top lens and the reticle plane of the imaging system. Distortion and field curvature correspond to the lowest order aberrations of an imaging system, namely field dependent tilt and lithographers-focus. Various techniques for in-situ measurement of distortion and field curvature have been developed. See M. Dusa et al., “In-house Characterization Technique for Steppers” Optical/Laser Microlithography II, 1989, SPIE Vol. 1088, p. 354; and D. Flagello, B. Geh entitled “Lithographic Lens Testing: Analysis of Measured Aerial Images, Interferometric Data and Photoresist Measurements”, SPIE Vol. 2726, p. 788, June 1996.
Techniques for the in-situ measurement of astigmatism have also been developed. See T. Brunner et al., “Characterization and Setup Techniques for a 5×Stepper”, Optical/Laser Microlithography V, SPIE Vol. 663, 1986, p. 106; and J. Kirk, entitled “Astigmatism and Field Curvature from Pin-Bars”, Optical/Laser Microlithography IV, SPIE Vol. 1463, p. 282, Mar. 6, 1991.
Techniques for analyzing aerial images and aberrations have also been developed. See A. Pfau et al., “A Two-Dimensional High-Resolution Stepper Image Monitor”, Optical/Laser Microlithography V, SPIE Vol. 1674, Mar. 11, 1992, p. 182; E. L. Raab et al, “Analyzing the Deep-UV Lens Aberrations Using Aerial Image and Latent Image Metrologies”, Optical/Laser Microlithography VII, SPIE Vol. 2197, Mar. 2, 1994, p. 550; and C. Huang, “In-situ Optimization of an I-Line Optical Projection Lens”, Optical/Laser Microlithography VIII, SPIE Vol. 2440, Feb. 22, 1995, p. 735.
Use of these, and other, techniques have allowed for rapid, unintrusive characterization of lens aberrations (see U.S. Pat. Nos. 5,828,455 and 5,978,985 both entitled “Apparatus, Method of Measurement and Method of Data Analysis for Correction of Optical System”, supra), illumination source (see U.S. Pat. No. 6,356,345 entitled “In-Situ Source Metrology Instrument and Method of Use”, supra) and lens distortion (see A. Smith et al., “Method & Apparatus for Self-Referenced Projection Lens Distortion Mapping”, U.S. Pat. No. 6,573,986, Jun. 2, 2003).
While these techniques are generally sufficient to characterize much of existing lithographic performance—especially for those lithographic exposure tools that are pushed near and beyond design specifications, both in pitch and resolution, it is also desirable to determine the lens, or imaging objective (IMO) transmission as a function of exit pupil transverse direction cosine (nx,ny)—at multiple field points—to allow for a more complete analysis and correction of the photolithographic exposure system. The output of such measurements would be the exit pupil transmission function T(nx,ny,xi,yi) at discrete points ((xi,yi)i=1:N) across the projection image field. Once known, basic details of the IMO such as effective numerical aperture as a function of field position, NA (xi,yi) and asymmetry of the numerical aperture, ΔNA (xi,yi) may be determined from T(nx,ny; xi,yi). In prior work, a method for determining across pupil transmission variation, (or across field pupil transmittance, APTV) using two-beam interference is discussed. See K. Sato et al., “Measurement of Transmittance Variation of Projection Lenses depending on the Light Paths using a Grating-Pinhole Mask”, SPIE Vol. 4346, 2001, pp. 379-386. Using this technique, a source illuminates a phase shift mask and is used to form images in resist patterns. The pitch of the line/space patterns on the phase shift mask is used to sample the transmission across the pupil. Known limitations of this interference method include: sensitivity to source uniformity; mask phase error; source sigma; and resist processing. See “Measurement of Transmittance Variation of Projection Lenses Depending on the Light Paths using a Grating-Pinhole Mask, supra; and K. Sato et al., “Impact of Across Pupil Transmittance Variation in Projection Lenses on Fine Device Pattern Imaging”, SPIE, Vol. 5040, 2003, pp. 33-44.
Thus, there is a need for more complete analysis and correction of a photolithographic exposure systems and for improved illumination systems and methods and apparatus to determine lens or imaging objective (IMO) transmission as a function of exit pupil transverse direction cosine (nx,ny) at multiple field points.